Dropping the Payload

What happens when a vesicle arrives at its destination? To release its contents, a vesicle's membrane must fuse with the membrane of its target. From the very first point of contact, the molecules in the two membranes begin rearranging, opening a pore between them.

“Nobody knows what the fusion pore is made of,” says Edwin Chapman, an HHMI investigator at the University of Wisconsin-Madison, noting one of the frustrations of studying membrane fusion. “Nobody knows whether it's all lipids or has proteins lining it. Everything about fusion pores is highly controversial and one of the major reasons is that they're very short-lived.”

Chapman's lab group aims to pull back the curtain on membrane fusion. In a recent paper, they looked at how the degree of bending of a membrane relates to its ability to fuse with another membrane. By using large vesicles that have, in essence, flat surfaces (think of how flat the spherical earth seems when you're standing on it), they demonstrated that membrane-bending proteins are required for a vesicle to fuse with this flat surface. A protein on the smaller, incoming vesicle penetrates the flat surface and bends it, making a protrusion for the vesicle to fuse with.

Computer simulations can explain why such a protrusion is energetically favorable to fusion. “With the onset of powerful computers, people started doing a lot of modeling and simulations of the membranes,” says Chapman. “It became very clear from that work that dimpling of the membrane really lowers the energy barrier for fusion.”

In Chapman's paper on membrane bending, which appeared in the August 21, 2009, issue of Cell, he shows that the same membrane-bending proteins that facilitate endocytosis—when a cell takes in materials from an outside vesicle—can, under certain circumstances, also facilitate exocytosis—the release of contents from the cell. It's a hint that the two processes are more similar than scientists have thought.

Chapman hopes his work improves understanding of the synapses between neurons. Here, vesicles full of neurotransmitters release their contents into the space between one neuron and the next, sending a chemical message along. To release their contents, these so-called synaptic vesicles must fuse with the outer membrane of the cell sending the message.

This neuronal fusion is at the core of what allows humans and animals to react to stimuli, think, make decisions, and remember. Chapman thinks that proteins involved in synaptic vesicle membrane sculpting could well be drug targets for diseases affecting the brain. He's exploring whether the tiny “fusion pore”, that opens between membranes during fusion, always results in complete merger of the vesicle and the target membrane, or whether fusion pores can sometimes simply close, in a process referred to as "kiss-and-run".

“Synaptic vesicles are very small, of a very precise size, with a very precise membrane composition,” says Pietro De Camilli, an HHMI investigator at the Yale School of Medicine who also studies the process. “Each vesicle must contain a certain amount of neurotransmitter.”

To control how much neurotransmitter is released, a cell doesn't change the size of its synaptic vesicles, says De Camilli; it changes the number of vesicles it releases. Each vesicle has just the right contents, a fact that De Camilli calls “utterly astounding.”

On each vesicle there are dozens of copies of some proteins, and only one or two of others, he says. “How does the cell make sure everything's there? How does it count?”

After fusion with the outer membrane, synaptic vesicles are rapidly regenerated by local membrane recycling. De Camilli's lab studies how the outer membrane is bent into small membrane buds that undergo fission to generate new synaptic vesicles. He has shown how this process correlates with chemical modification of membrane lipids. By modifying lipids, the overall charge of a membrane can be altered. This, in turn, makes proteins interact differently with the membrane. De Camilli discovered in 2007 that Lowe syndrome, a form of mental retardation in boys, results from a malfunction in membrane recycling processes due to modifications in one part of this machinery. It's one more sign of how important the intricate interplay of membranes is to the health of an organism.